The two-dimensional (2D) hexagonal boron nitride (hBN) hosting optically active defects has recently emerged as a promising system for quantum applications [1]}, [2]}. Its 2D nature allows hBN transferred and integrated with solid devices like cavities and other heterogeneous materials in a controllable way so that the defects can have controllable interaction.[3]}, [4]}, [5]}
Especially, for spin defects allowing optical initialization, readout and spin manipulation, the nanoscale proximity of the spin defects to target samples may allow for high-resolution quantum-sensing.
The potential spin defects include negatively-charged boron vacancy (\( V_B^- \) ) defects [6]}, [7]}, [8]}, [9]}, [10]}, carbon-related defects[8]}, [7]} and nitrogen-related defects[7]}.
For \( V_B^- \)  defects, recent works have confirmed that it is a spin three-level system and revealed structure of the ground state (GS), which can be potentially used for sensing temperature, pressure, and magnetic field [6]}, [15]}, [9]}. However, the excited state (ES) structure as well as the dynamics of excitation-emission cycles are not yet fully understood [17]}, [18]}. Understanding this excited-state spin level is of critical importance for better control of \( V_B^- \)  centers spin state and quantum sensing application.
